Test strips are often used to measure the presence and/or concentrations of selected analytes in test samples. For example, a variety of test strips are used to measure glucose concentrations in blood to monitor the blood sugar level of people with diabetes. These test strips include a reaction chamber into which a reagent composition has been deposited. Current trends in test strips require smaller test samples and faster analysis times. This provides a significant benefit to the patient, allowing the use of smaller blood samples that can be obtained from less sensitive areas of the body. Additionally, faster test times and more accurate results enable patients to better control their blood sugar level.
In connection with smaller sample volumes, it is known to provide test strips having a sufficiently small reaction chamber such that sample fluid is drawn therein by capillary action, which is a phenomenon resulting from the surface tension of the sample fluid and the thermodynamic tendency of a liquid to minimize its surface area. For example, U.S. Pat. No. 5,141,868 discloses a test strip having a cavity sized sufficiently small to draw sample liquid therein by capillary action. The cavity is defined by two parallel plates spaced about 1 mm apart by two epoxy strips extending lengthwise along lateral sides of the plates. The cavity is open at both ends, one of which receives the sample, and the other of which allows air to escape. The cavity includes an electrode structure and carries a coating of a material appropriate to the test to be performed by the test strip.
Various other test strip designs include capillary cavities that draw sample fluid therein and include vent openings to allow air to escape. As one should appreciate, capillary channels in current test strip designs are typically very small and are continually being designed smaller to reduce the amount of sample needed for testing. However, the smaller the capillary entrance width, the more difficult it becomes to accurately apply (or “target”) a small sample volume to the capillary of the test strip. Targeting is even more important in segments of the demographic with impaired vision and/or reduced dexterity because it is more difficult for this segment to accurately align their fingers with the dosing edge of a test strip. Furthermore, the sample fluid sometimes undesirably hesitates before being drawn into the capillary, a phenomenon referred to as “dose hesitation.” It would be desirable to overcome the difficulties associated with small capillaries in test strip design.
A limitation of electrochemical methods of measuring the concentration of a chemical in blood is the effect of confounding variables on the diffusion of analyte and the various active ingredients of the reagent. Examples of limitations to the accuracy of blood glucose measurements include variations in blood composition or state (other than the aspect being measured). For example, variations in hematocrit (concentration of red blood cells) can effect the signal generation of a blood sample. The utility of a reported blood glucose response after a short test time is questionable in applications where the results are not compensated for other sample variables or interferents such as hematocrit and temperature.
With respect to hematocrit in blood samples, prior art methods have relied upon the separation of the red blood cells from the plasma in the sample, by means of glass fiber filters or with reagent films that contain pore-formers that allow only plasma to enter the films, for example. Separation of red blood cells with a glass fiber filter increases the size of the blood sample required for the measurement, which is contrary to test meter customer expectations. Porous films or membranes are only partially effective in reducing the hematocrit effect, and must be used in combination with increased delay time and/or specialized measurement techniques to achieve the desire accuracy. Thus, it is desirable to manufacture a biosensor that is capable of reducing hematocrit interference in an easy, simple, and cost-effective manner.
Adhesives are typically used to join or seal together various layers of test strips. For high volume production of test strips, adhesives can be a significant cost both as a raw material and especially with respect to manufacturing costs. For instance, several manufacturing processes can be detrimentally affected by the adhesives. As an example, slitters, which are used to cut a web to form the strips, can be gummed up over time as a result of a build up of adhesive sawdust. As a result, the slitters must be shut down periodically for cleaning. This periodic interruption of production results in lower production rates. Thus, it is desirable to manufacture test strips in an inexpensive manner.